Add XML Base and xml:id or xml:lang convenience semantics

[FlorianRappl]

Description

Introduce convenience APIs and behavior for common XML namespace-defined attributes:

• effective base URI resolution via xml:base
• xml:id handling for ID semantics and lookups
• effective language resolution through xml:lang inheritance

The goal is to reduce repeated custom logic in client code and improve standards-aligned behavior out of the box.

Background

User scenarios include:

• resolving relative links in XML content where base URI changes in subtree context
• using xml:id for robust anchors and cross-references
• applying inherited language metadata in content processing pipelines

Many XML consumers expect these semantics to be readily available.

Specification

Relevant specifications:

• XML Base 1.0 (Second Edition)
• xml:id Version 1.0
• W3C XML 1.0 (Fifth Edition), section 2.12 Language Identification